1. Field of the Invention
The invention relates to a contactless power transmission system for transmission of power between a rotating part and a stationary part, in particular between a rotating part and a stationary part of a computer tomograph, and also to a method for transmitting power via a rotating power transmission device.
2. Description of the Relevant Art
With rotating units such as radar units or also computer tomographs, and also with linearly movable units such as crane and conveyor units, it is necessary to transmit electrical power between units that are movable relative to each other.
Non-contacting inductive rotary joints are an advantageous substitute for the known mechanical slip-rings for transmission of electrical energy. In inductive transmission technology a coupling between rotatable units is effected with magnetic fields without contact. This has an advantage over mechanical slip-rings in that torque, wear, and maintenance are minimized. Furthermore, the surroundings of the rotary joints are not polluted by carbon dust.
Inductive rotary joints, as disclosed in U.S. Pat. No. 7,197,113 have at least one winding on each of the rotatable units. Furthermore, an iron core or ferrite core for concentrating the magnetic field may be provided on the rotor as well as on the stator, or also on both parts. An alternating current signal is fed into a winding of one of the parts while another winding on the other part feeds a load.
With a conductively coupled slip-ring a constant voltage can be easily transferred from a constant voltage power supply at the stationary side to a load at the rotating side. Due to the conductive connection of low ohmic resistance through the slip-ring, the output voltage will correspond to the input voltage, except for minor deviations. Owing to the low resistance of the slipping, a low and usually negligible voltage drop depending on the load current is obtained.
At inductively coupled rotary joints an equivalent circuit diagram of the transmission device includes a stray inductance as a series inductance between the input side and the load side. This results in a series reactance between input and load. This stray inductance depends on the intrinsic inductance of the joint and, in particular, on the coupling factor. Especially with inductive rotary joints of large dimensions, it is often possible to obtain only a small coupling factor which, in addition, frequently fluctuates with the positions of the rotatable units relative to each other. Thus, for example, the coupling factor decreases with increase of an air-gap between the iron cores that are rotatable relative to each other. The stray inductance and accordingly its series reactance then increase accordingly. This leads to significant changes in load voltage. Furthermore with a lower coupling factor changes of the load resistance result in stronger changes of load voltage.
In order to transmit higher power via the rotary joint despite this stray inductance, the stray inductance is used in suitable circuits like a discrete inductance. Its use would be, for example, as a storage inductance, or also as a resonance inductance. In the case of a resonance inductance, the inductance can be supplemented for example with a series capacity to form a series resonance circuit, or with a parallel capacity to form a parallel resonance circuit. Of course, more complex resonating structures also may be obtained. A rotary transmission device of this kind having resonance circuits is disclosed for example in the International patent application publication WO 98/32217 A1. One of the most serious problems in such resonance circuits is the energy stored in the resonance elements e.g. the inductance and the capacitance. This energy makes a quick control or change of the output parameters (Voltage, Current) very difficult. If the output voltage should be lowered then first the energy from the resonance circuit must be discharged. Otherwise before increasing the output voltage, energy must be charged into the resonance circuit. Another problem is that a data link is required for feedback of the measured output voltage to the primary side.
In order to achieve constant supply to the output side, and to prevent a destruction of the connected components, it is necessary to control at least one of the electrical parameters on the output side. For low power transmission, a separate controller such as a voltage controller that is constructed to be a series regulator or also a switching controller may be used at the output side. For higher power, at least one sensor for at least one of these electrical parameters should be provided on the output side. This sensor determines the magnitude of this electrical parameter and signals it to the alternating signal source on the input side. Now another electrical parameter such as for example current, voltage or also the frequency on the input side can be controlled with a control amplifier so that the electrical parameter at the output side is kept constant. A technology of this kind is known for conventional switching power supplies and is also regularly applied and disclosed in U.S. Pat. No. 7,054,411. With rotary joints there is the problem that information from a sensor must be transmitted from the output side to the input side, i.e. between two units that are rotatable relative to each other. This requires a further rotary joint operating in an opposite transmission direction from that of the inductive power transmitter. However, often no mechanical construction space is available for an additional rotary joint. This may also increase costs.
The problems presented here increase with increase of size of the rotary joint. Thus, with compact units having diameters of a few centimeters it is still possible to use precise bearings with tolerances below 0.1 mm. With this, it is possible to achieve, for example, a precise air-gap of 0.2 mm, and a fluctuation during rotation in a range of 0.2 mm to 0.3 mm. With large units having diameters larger than 1 meter, as used for example in computer tomographs, the tolerances are already in a range of a few millimeters, and are partly greater than 5 mm. Thus, in a case like this the air-gap would vary between 1 and 6 mm, depending on position and operating conditions. This leads to a substantially larger stray inductance which, in addition, fluctuates substantially more strongly.
Another problem is control of the power delivered through the rotating transformer. Prior art uses AC generators fed by the AC line voltage as it is supplied by a power line. To control the power to be delivered to the load, the frequency of the AC generator is controlled. Maximum power can be transferred at the resonance frequency of the rotating transformer and its resonance capacitor. At lower or higher frequencies the power transfer is reduced. The problem is that with lower transferred power the reactive power and therefore the reactive currents in the circuit increase. Furthermore switching of currents which are out of phase with voltage as they occur at frequencies off the resonance frequency is difficult and leads to higher power losses in the switching circuit.